The trend in the design of power systems for electronic assemblies has been toward the use distributed power architectures. In a distributed power system, many small, individual board mounted power supply modules are used in place of a few larger, centralized power supplies. Such a distributed power architecture could be used to power a diverse variety of electronic assemblies, including, for example, a computer work station, a file server, or a telecommunications switching system. Each of the board mounted power modules is conveniently located close to the circuitry being powered, often with one or more power modules located on each circuit card in the electronic assembly. It is advantageous to minimize the size of the individual power modules, as the circuit card space used by the power module could be used, for example, for additional circuitry to increase the processing throughput of a computer card, or to increase the switching capacity of a telecommunications card. Therefore, a power module which has a relatively large footprint area is generally considered undesirable. Additionally, many electronic assemblies employ circuit card to circuit card spacing of less than one inch. Minimizing the card to card spacing allows for a denser assembly, which advantageously increases throughput or capacity. It is therefore desirable to provide a power module with the additional attribute of a low height profile.
Consequently, the trend in the design of power supply modules has been toward achieving increased output power along with a lower height profile and a smaller footprint area, thereby increasing power density. However, any improvements in power level, power density or profile cannot be at the expense of the thermal and electrical characteristics of the overall power module and its constituent internal devices.
A power supply module is conventionally constructed as a unitary, encapsulated package, with one or more rows of leads to connect the module to a circuit card, and a metallic case containing attachment mounts for an external heatsink. The internal construction of the power module often comprises one or more power devices (e.g., transistors and diodes) in thermal communication with the metal case, one or more magnetic devices (e.g., transformers and inductors) providing electrical isolation and energy storage, and one or more circuit boards containing electronic components to, among other things, provide control and monitoring functions.
Using a DC/DC power converter as an example, a simplified internal structure of a conventional power module package will hereinafter be described. The packaged power devices and magnetic devices that must be thermally managed due to high power dissipation are mounted onto a metal circuit board employing an insulated metal substrate technology, such as a THERMAL CLAD substrate manufactured by the Bergquist Corporation of Minneapolis, Minn. The metal circuit board also contains components for filtering and control. The remainder of the filtering and control components are mounted to a conventional FR4 printed circuit board, which is mechanically and electrically connected to the metal circuit board to facilitate communication and power flow between various parts of the power supply module. Leads allowing for communication and power flow between the power module and the external circuit card are mechanically and electrically fastened to one or both of the circuit boards in the power module. The power module is cased in a plastic or metallic enclosure which receives an encapsulant operable for protecting the internal components from contaminants and perhaps to improve heat flow between the internal components and the enclosure. The enclosure also contains means for attaching an external heatsink.
While the aforementioned internal structure provides a viable alternative for packaging a power supply module, it endures several limitations. First, it requires two printed wire boards to accommodate components and interconnection traces, e.g., the metal circuit board and the FR4 circuit board. Such a two board design increases the cost of the module and increases complexity of assembly. Second, the module employed discrete magnetic devices and did not incorporate planar magnetics into the printed circuit board. The use of planar magnetics can reduce the size of the design by allowing components to be placed directly on top of transformer and inductor windings, thus effectively making double use of the available internal space. Planar magnetics can also reduce the cost of the module. Third, the packaged power density is low. Finally, the design requires several iterative manual steps that are not consistent with a mass producible and cost effective design for a power supply module.
Accordingly, what is needed in the art is a power supply module which simultaneously achieves the requirements of high packaged power density and cost effectiveness, without sacrificing the thermal and electrical characteristics of the device. Preferably, the aforementioned power supply module should present a user with the minimum number of features and functions required to provide tight tolerance remote voltage regulation, protection during a fault or trouble condition, and external control of the power supply module.